This invention relates to a method and apparatus for controlling static charges and particularly to a method and apparatus for producing an ionized field and controlling the balance and magnitude of the directional conductivity of the field in order to control static on film and other dielectric material.
It has been found that when using conventional static control devices, low level static charges appear to be left on films or other dielectric material. These low level charges were frequently responsible for subsequent processing problems, which may or may not have been recognized as being caused by static electricity. For example, in applications wherein particulate materials, such as coffee, are being packaged in a plastic bag, the application of high voltage AC for ionization purposes to reduce static charges on the plastic film imparts a negative charge to the film which attracts the particulate materials which usually have a positive charge. Accordingly, in such packaging applications, there is a tendency for the particulate materials to adhere to the film after the film passes by a static control device, thereby adversely affecting the packaging operation by preventing the proper sealing of the film to form an enclosed bag.
In theory, dielectrics exposed to high voltage AC ionized gas fields would be expected to leave the field in a neutral condition since the areas under the positive and negative segments of the sinusoidal AC voltage wave form have an algebraic sum of zero. This should yield a neutral ionized field which exhibits equal conductivity in both directions. In practice, however, such ionized gas fields nearly always show directional conductivity which heretofore has not been easily controllable. Directional conductivity occurs when the ionized gas field conducts more in one direction than another. This can easily be measured by using commercially available equipment.
Additionally, in the processing of film or other dielectric material which is affected by static charges, undesirable static charges are frequently imparted to the film or other material as a result of passage past rollers or other parts of the processing equipment. Furthermore, because of space limitations, it is sometimes difficult to place conventional static control equipment at the location where static control is desired.
The present invention is directed to a method and apparatus for overcoming the foregoing problems to allow control of ion field balance and/or directional conductivity and permit management of the field's final effect with respect to processes involving ionized gas fields, such as static control applications.